1. Field of the Invention
The present invention relates to chilled water systems that include variable flow pumps for circulating the chilled water through multiple chillers. More specifically, the present invention relates to a method of controlling such a system.
2. Description of Related Art
A chiller is an assembly of refrigerant components arranged in a circuit for cooling water. The chilled water is typically pumped to a number of remote heat exchangers or system coils for cooling various rooms or areas within a building.
In some cases, the water may be cooled by a chiller system comprising two or more chillers. When the cooling demand is low, only one chiller of the system may need to operate, and the operating chiller""s capacity may be controlled to match the demand. The cooling demand is often determined by sensing the temperature of the chilled water discharged from the chiller system and comparing the sensed temperature to a predetermined target temperature. If the cooling demand is beyond a single chiller""s maximum capacity, one or more additional chillers may need to be energized. Then, the operating chillers are controlled so the system""s total capacity (sum of the chillers"" individual capacities) meets the cooling demand.
Meanwhile, the chilled water is pumped at a flow rate that is adequate for each individual chiller and is delivered at a pressure sufficient to meet the needs of the system coils. This can be accomplished by pumping the chilled water with variable speed pumps and/or controlling a bypass valve to convey a portion of the discharged chilled water back to the suction side of the pumps.
Overall, controlling a chiller system can become quite involved. This is due to the difficulty of coordinating the control of several diverse chiller components, such as multiple chillers of varying capacity, multiple variable speed pumps, and a bypass valve. Moreover, the system components must operate to satisfy various needs, such as meeting the cooling demand, providing sufficient water pressure for the system coils, and providing adequate water flow through the chillers. A need to minimize the power consumption of the chillers and the chilled water pumps further complicates the controls of chiller systems. Although controls of such systems do exist, their actual control schemes may limit their use or effectiveness in certain applications, and their complexity may make them difficult to understand, install and service. Since many chiller installations have unique system requirements, there is a need for a more adaptable, straightforward control scheme for controlling chiller systems with variable speed chilled water pumps.
It is an object of the present invention to coordinate the operation of multiple chillers, multiple variable speed pumps, and a bypass valve to meet a cooling demand.
Another object of some embodiments of the invention is to energize a second chiller in response to a cooling demand exceeding that what can be met by a first chiller, and de-energizing the second chiller upon the cooling demand decreasing to a level below the first chiller""s maximum capacity.
Another object of some embodiments is to operate two chillers in unison, whereby the chillers operate at the same capacity with respect to a percentage of their maximum capacity.
Another object of some embodiments is to operate two pumps at the same speed, but vary their speed to achieve a certain discharge pressure or pressure differential.
Another object, for a chiller system having two variable speed water pumps, is to maintain sufficient water flow through two chillers by opening a bypass valve that is in parallel flow relationship with the chillers.
Another object of some embodiments is to vary the speed of two pumps in response to sensing a pressure differential across a remote heat exchanger coil.
One or more of these objects are provided by a chiller system that includes two variable speed pumps that pump water through a first chiller and a second chiller for cooling the water. A control energizes the second chiller in response to a cooling demand exceeding that what can be met by the first chiller operating alone, and de-energizes the second chiller upon the cooling demand decreasing to a level below the first chiller""s maximum capacity.
The present invention provides a method of controlling a chiller system that includes a first chiller and a second chiller through which water can be pumped to meet a cooling demand. The method comprises: pumping the water through the first chiller at a first flow rate to meet the cooling demand; increasing the cooling demand; in response to increasing the cooling demand, pumping the water through the first chiller at a second flow rate that is less than the first flow rate; and in response to increasing the cooling demand, pumping the water through the second chiller at a third flow rate, wherein the first flow rate is substantially equal to a sum of the second flow rate plus the third flow rate. The present invention also provides, with respect to the water, piping the first chiller and the second chiller in parallel flow relationship with a heat exchanger that is spaced apart from the first chiller and the second chiller, whereby the water is conveyed to the heat exchanger via a supply line and is conveyed from the heat exchanger via a return line; sensing a water pressure differential between the supply line and the return line; and controlling the first flow rate, the second flow rate and the third flow rate in response to sensing the water pressure differential.
The present invention further provides a method of controlling a chiller system that includes a first chiller and a second chiller for meeting a demand for chilled water, wherein the first chiller is selectively operable at a first full load and a first range of partial loads, and the second chiller is selectively operable at a second full load and a second range of partial loads. The chiller system further includes a chilled water circuit, a first pump for forcing the chilled water through the first chiller at a first flow rate that may vary, a second pump for forcing the chilled water through the second chiller at a second flow rate that may vary, a bypass valve, a first heat exchanger, and a second heat exchanger. The chilled water circuit connects the first chiller, the second chiller, the bypass valve, the first heat exchanger, and the second heat exchanger in parallel flow relationship with respect to the flow of chilled water. The method comprises increasing the demand for chilled water; in response to increasing the demand for chilled water, changing the operation of the first chiller from operating at the first full load to operating within the first range of partial loads; in response to increasing the demand for chilled water, reducing the first rate at which the first pump forces chilled water through the first chiller; and in response to increasing the demand for chilled water, energizing the second chiller to begin operating the second chiller in the second range of partial loads. The present invention yet further provides, via a supply line of the chilled water circuit, conveying the chilled water to the first heat exchanger and the second heat exchanger; via a return line of the chilled water circuit, conveying the chilled water from the first heat exchanger and the second heat exchanger; sensing a water pressure differential between the supply line and the return line; and varying the first flow rate and the second flow rate in response to sensing the water pressure differential.
The present invention still further provides a method of controlling a chiller system that includes a first chiller and a second chiller for meeting a demand for chilled water. The first chiller is selectively operable at a first full load and a first range of partial loads, and the second chiller is selectively operable at a second full load and a second range of partial loads. The chiller system further includes a chilled water circuit, a first pump for forcing the chilled water through the first chiller at a first flow rate that may vary, a second pump for forcing the chilled water through the second chiller at a second flow rate that may vary, a bypass valve, a first heat exchanger, and a second heat exchanger. The chilled water circuit connects the first chiller, the second chiller, the bypass valve, the first heat exchanger, and the second heat exchanger in parallel flow relationship with respect to the flow of chilled water. The method comprises establishing a chilled water temperature target; establishing a chilled water pressure target; selectively operating the chiller system in a high demand mode and a low demand mode to meet the chilled water temperature target; in the low demand mode, leaving the second chiller inactive while selectively operating the first chiller in the full load and the first range of partial loads to meet the chilled water temperature target; in the low demand mode, leaving the second pump inactive while modulating the pressure of the chilled water by controlling the operation of the first pump to meet the chilled water pressure target; in the high demand mode, operating the first chiller at a first partial load while operating the second chiller at a second partial load; and in the high demand mode, modulating the pressure of the chilled water by controlling the operation of the first pump and the second pump to meet the chilled water pressure target.
The present invention additionally provides a method of controlling a chiller system that includes a first chiller and a second chiller for meeting a demand for chilled water. The first chiller is selectively operable at a first full load and a percent of the first full load ranging from zero to one hundred percent, and the second chiller is selectively operable at a second full load and a percent of the second full load ranging from zero to one hundred percent. The chiller system further includes a chilled water circuit, a first pump for forcing the chilled water through the first chiller at a first flow rate that may vary, a second pump for forcing the chilled water through the second chiller at a second flow rate that may vary, a bypass valve, a first heat exchanger, and a second heat exchanger. The chilled water circuit connects the first chiller, the second chiller, the bypass valve, the first heat exchanger, and the second heat exchanger in parallel flow relationship with respect to the flow of chilled water. The method comprises establishing a chilled water temperature target; establishing a chilled water pressure target; selectively operating the chiller system in a high demand mode and a low demand mode to meet the chilled water temperature target; in the low demand mode, leaving the second chiller inactive while operating the first chiller to meet the chilled water temperature target; in the low demand mode, leaving the second pump inactive while modulating the pressure of the chilled water by controlling the operation of the first pump to meet the chilled water pressure target; in the low demand mode, modulating the pressure of the chilled water by controlling the operation of the first pump and the second pump to meet the chilled water pressure target; in the high demand mode, modulating the first chiller at a percentage of the first full load; and in the high demand mode, modulating the second chiller at a percentage of the second full load and in unison with the first chiller, whereby the percentage of the first full load is substantially equal to the percentage of the second full load.
The present invention moreover provides a chiller system. The system comprises a first chiller wherein the first chiller is selectively operable at a first full load and a first range of partial loads; and a second chiller for meeting a demand for chilled water wherein the second chiller is selectively operable at a second full load and a second range of partial loads. The system also comprises a first pump for forcing the chilled water through the first chiller at a first flow rate that may vary, a second pump for forcing the chilled water through the second chiller at a second flow rate that may vary; a bypass valve; a first heat exchanger; a second heat exchanger; and a chilled water circuit. The chilled water circuit connects the first chiller, the second chiller, the bypass valve, the first heat exchanger, and the second heat exchanger in parallel flow relationship with respect to the flow of chilled water; control circuitry or logic establishing a chilled water temperature target; control circuitry or logic establishing a chilled water pressure target; control circuitry or logic selectively operating the chiller system in a high demand mode and a low demand mode to meet the chilled water temperature target. The system further comprises, in the low demand mode, leaving the second chiller inactive while selectively operating the first chiller in the full load and the first range of partial loads to meet the chilled water temperature target; control circuitry or logic, in the low demand mode, leaving the second pump inactive while modulating the pressure of the chilled water by controlling the operation of the first pump to meet the chilled water pressure target; control circuitry or logic, in the high demand mode, operating the first chiller at a first partial load while operating the second chiller at a second partial load; and control circuitry or logic, in the high demand mode, modulating the pressure of the chilled water by controlling the operation of the first pump and the second pump to meet the chilled water pressure target.
The present invention still further provides a chiller system. The system includes a first chiller where the first chiller is selectively operable at a first full load and a percent of the first full load ranging from zero to one hundred percent; a second chiller for meeting a demand for chilled water where the second chiller is selectively operable at a second full load and a percent of the second full load ranging from zero to one hundred percent; a first pump for forcing the chilled water through the first chiller at a first flow rate that may vary; and a second pump for forcing the chilled water through the second chiller at a second flow rate that may vary. The system also includes a bypass valve; a first heat exchanger; a second heat exchanger; and a chilled water circuit wherein the chilled water circuit connects the is first chiller, the second chiller, the bypass valve, the first heat exchanger, and the second heat exchanger in parallel flow relationship with respect to the flow of chilled water. The system also includes a controller establishing a chilled water temperature target and a chilled water pressure target, the controller selectively operating the chiller system in a high demand mode and a low demand mode to meet the chilled water temperature target. In the low demand mode, the controller leaves the second chiller inactive while operating the first chiller to meet the chilled water temperature target; in the low demand mode, the controller leaves the second pump inactive while modulating the pressure of the chilled water by controlling the operation of the first pump to meet the chilled water pressure target; in the low demand mode, the controller modulates the pressure of the chilled water by controlling the operation of the first pump and the second pump to meet the chilled water pressure target; in the high demand mode, the controller modulates the first chiller at a percentage of the first full load; and in the high demand mode, the controller modulates the second chiller at a percentage of the second full load and in unison with the first chiller. The percentage of the first full load is substantially equal to the percentage of the second full load.